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#include <fstream>
#include<iomanip>
#define ab(a) ((a < 0) ? -a : a)
#define FOR(i,a,b) for(int i = a; i <= b; i++)
#define NMAX 100005
using namespace std;
double a[NMAX],b[NMAX],aria;
ifstream fin("aria.in");
ofstream fout("aria.out");
int N;
int main()
{
fin>>N;
FOR(i,1,N)
{
fin>>a[i]>>b[i];
}
a[N+1] = a[1];
b[N+1] = b[1];
FOR(i,1,N)
{
aria+=a[i]*b[i+1]-a[i+1]*b[i];
}
aria = (double) aria/2.0;
fout<<setprecision(5)<<(double)ab(aria)<<'\n';
return 0;
}